[AIRFLOW-XXX] Fix development packages installtion instructions (#6942)

LOCAL_VIRTUALENV.rst

@@ -91,26 +91,51 @@ To use your IDE for Airflow development and testing, you need to configure a vir
 environment. Ideally you should set up virtualenv for all Python versions that Airflow
 supports (3.5, 3.6).
 
-Consider using one of the following utilities to create virtual environments and easily
-switch between them with the ``workon`` command:
+To create and initialize the local virtualenv:
 
-- `pyenv <https://github.com/pyenv/pyenv>`_
-- `pyenv-virtualenv <https://github.com/pyenv/pyenv-virtualenv>`_
-- `virtualenvwrapper <https://virtualenvwrapper.readthedocs.io/en/latest/>`_
+1. Create an environment with one of the two options:
 
-To create and initialize the local virtualenv:
+   - Option 1: consider using one of the following utilities to create virtual environments and easily switch between them with the ``workon`` command:
+
+    - `pyenv <https://github.com/pyenv/pyenv>`_
+    - `pyenv-virtualenv <https://github.com/pyenv/pyenv-virtualenv>`_
+    - `virtualenvwrapper <https://virtualenvwrapper.readthedocs.io/en/latest/>`_
+
+    ``mkvirtualenv <ENV_NAME> --python=python<VERSION>``
+
+   - Option 2: create a local virtualenv with Conda
 
-1. Create an environment as follows:
+    - install `miniconda3 <https://docs.conda.io/en/latest/miniconda.html>`_
 
-   ``mkvirtualenv <ENV_NAME> --python=python<VERSION>``
+    .. code-block:: bash
+
+      conda create -n airflow python=3.6
+      conda activate airflow
 
 2. Install Python PIP requirements:
 
-   ``pip install -e ".[devel]"``
+   .. code-block:: bash
+
+    pip install -U -e ".[devel,<OTHER EXTRAS>]" # for example: pip install -U -e ".[devel,gcp,postgres]"
+
+Note: when you first initialize database (the next step), you may encounter some problems.
+This is because airflow by default will try to load in example dags where some of them requires dependencies ``gcp`` and ``postgres``.
+You can solve the problem by:
+
+- installing the extras i.e. ``[devel,gcp,postgres]`` or
+- disable the example dags with environment variable: ``export AIRFLOW__CORE__LOAD_EXAMPLES=False`` or
+- simply ignore the error messages and proceed
+
+*In addition to above, you may also encounter problems during database migration.*
+*This is a known issue and please see the progress here:* `AIRFLOW-6265 <https://issues.apache.org/jira/browse/AIRFLOW-6265>`_
 
 3. Create the Airflow sqlite database:
 
-   ``airflow db init``
+   .. code-block:: bash
+
+    # if necessary, start with a clean AIRFLOW_HOME, e.g.
+    # rm -rf ~/airflow
+    airflow db init
 
 4. Select the virtualenv you created as the project's default virtualenv in your IDE.
 
@@ -122,21 +147,6 @@ Simply enter the Breeze environment by using ``workon`` and, once you are in it,
 
   ./breeze --initialize-local-virtualenv
 
-Optional - create a local virtualenv with conda
------------------------------------------------
-
-- install `miniconda3 <https://docs.conda.io/en/latest/miniconda.html>`_
-
-.. code-block:: bash
-
-  conda create -n airflow python=3.6
-  conda activate airflow
-  pip install -U -e ".[devel,gcp,postgres,ssh]"
-
-  # if necessary, start with a clean AIRFLOW_HOME, e.g.
-  # rm -rf ~/airflow
-  airflow db init
-
 Running Tests
 -------------
 

STATIC_CODE_CHECKS.rst

@@ -133,9 +133,9 @@ them with ``sudo apt install``, on macOS - with ``brew install``.
 
 The current list of prerequisites is limited to ``xmllint``:
 
-- on Linux, install via ``sudo apt install xmllint``;
+- on Linux, install via ``sudo apt install libxml2-utils``;
 
-- on macOS, install via ``brew install xmllint``.
+- on macOS, install via ``brew install libxml2``.
 
 Enabling Pre-commit Hooks
 .........................